Selectivity() can't correctly recognize an index can cover which expressions

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Development Task

CREATE TABLE `t` (
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL,
  `c` int(11) DEFAULT NULL,
  KEY `iab` (`a`,`b`),
  KEY `ic` (`c`)
);

Add a breakpoint in getMaskAndRanges() and run this SQL:

select * from t where a >= 2 and a <= 2 and b >= 20 and b <= 20 and c > 10;

When handling index iab, you'll find the mask is 0, which means this index doesn't cover any expressions. However, this is wrong, in fact, index iab covers the first 4 expressions. Eventually, this makes GetUsableSetsByGreedy() chooses statistics of column a, column b, and index ic to calculate selectivity, which should have been index iab and index ic by design.

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/sig planner

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/assign time-and-fate